This invention relates to a field measuring device for measuring magnetic fields and electric currents by making use of the Faraday effect.
The Faraday effect refers to a phenomenon wherein the plane of polarization of light passing through an optical material rotates in proportion to the strength of a magnetic field applied in the direction of the optic axis. Furthermore, an element consisting of an optical material having such property is referred to as a Faraday cell.
The Faraday effect may be understood from the example illustrated in FIG. 1, in which light 2, whose direction of polarization is taken as the y-direction, is incident upon an optical material 1 of length l. When a magnetic field H is applied in the same direction as the light 2, the direction of polarization of the emergent light is rotated by an angle .theta. proportional to H and l. This is illustrative of the Faraday effect. The angle .theta. is represented by: EQU .theta.=Ve.multidot.l.multidot.H (1)
where the constant of proportionality Ve is referred to as Verdet's constant.
In FIG. 2, which is useful in describing an arrangement for measuring a magnetic field or a current, light emitted from a light source 6 is converted into linearly polarized light by a polarizer 7. Upon passage through the optical material 1, the linearly polarized light is intercepted by an analyzer 8 which isolates the change in optical intensity caused by the change in direction of polarization. The light which emerges from the analyzer 8 is converted into an electrical signal V.sub.out by a photodetector 9. When the angle defined by the polarizer 7 and analyzer 8 is set to 45 degrees, the electrical signal V.sub.out delivered by the photodetector is expressed by: EQU V.sub.out =1/2.multidot.K.multidot.(1+sin 2.theta.) (2)
where K is a constant decided by the intensity of the incident light and the sensitive of the photodetector 9. If we extract the change which is due to the applied magnetic field, then Eq. (2) will give us: ##EQU1## Thus, when .theta. is small, an output proportional to the applied magnetic field H can be obtained. It is well known that placing an apparatus of this kind below a conductor 10 carrying an electric current makes it possible to measure the magnetic field produced by the current and, hence, the magnitude of the current itself.
Conventionally, lead glass is the material most widely used to fabricate a Faraday cell. The reason is that lead glass exhibits a comparatively large Verdet's constant (0.093 min/Oe.cm for a wavelength .lambda. of 633 nm.), and because of its intrinsically favorable temperature stability, which is one of the characteristics of diamagnetic glass. It has also become possible to manufacture glass which, owing to the addition of other metal elements (such as Tb), exhibits a Verdet's constant approximately twice that of lead glass. Such glass is paramagnetic, however, and therefore is disadvantageous in that Verdet's constant varies inversely with absolute temperature T, giving an unfavorable temperature characteristic.
Accordingly, one object of the present invention is to provide a current and field measuring device having excellent characteristics acquired through use of a Faraday cell the sensitivity of which is higher than that of lead glass but which also exhibits highly favorable temperature stability.
To attain the foregoing object, the Faraday material employed in the invention is bismuth silicon oxide (Bi.sub.12 SiO.sub.20) or bismuth germanium oxide (Bi.sub.12 GeO.sub.20), hereinafter referred to as BSO and BGO, respectively.
That BSO and BGO exhibit a comparatively large Verdet's constant (0.2 min/Oe.cm in both instances) is known from Applied Physics Letters, vol. 16, No. 5, (1970), p. 201. However, both of these compounds have an optical rotatory power which causes the direction of polarization to rotate even in the absence of an applied magnetic field, and the optical rotatory power involves a degree of temperature dependence. Moreover, both compounds exhibit an electrooptic effect, namely the Pockels effect, and thus are influenced by electric fields. For these and other reasons, BSO and BGO have not heretofore found use in Faraday cells.
A second object of the present invention, therefore, is to devise an expedient for eliminating the foregoing disadvantages to provide a field and current measuring device which exhibits the excellent temperature stability intrinsically possessed by diamagnetic substances, but which employs BSO or BGO to exploit the advantage offered by these materials, namely a sensitivity that is twice that of lead glass.
To this end, the inventors have discovered a method capable of sensing the influence solely of the Faraday effect, free of temperature dependence, by devising means for cancelling optical rotatory power (.sigma..sup.o /mm) as well as the change thereof with temperature (.DELTA..sigma./.DELTA.T). Accordingly, the second object of the invention is attained by providing a field and current measuring device to which the aforementioned discovering is applied.
Other features and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings.